Transcript lect21a.ppt

Superscalar Processing
Concepts (Preview)
CS 740
October 30, 2007
• Your Mission
• Complications
Your Mission
Design a single processor that will achieve a
millionfold speedup over our simple 5-stage pipeline.
Example: program that executes 1 Billion instructions
• 5-stage pipeline:
• new processor:
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1,000,000,000 cycles
1,000 cycles
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What About “Superpipelining”?
I.e. what if we design a 5,000,000 stage pipeline?
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How Many Functional Units?
How many instructions are executing at once?
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What About Branches?
Roughly 1 out of every 6 instructions is a conditional
branch.
How do we look far enough into the future?
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Branch Prediction
Two Parts:
(1) what is the target address?
(2) will the branch be taken or not?
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Executing Beyond Unresolved Branches
What if we execute instructions that never should
have been executed in the first place?
• What about their side-effects?
This is called “control speculation”.
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What about Data Dependences?
• Register renaming
• advantages?
• Dependences in the memory system
• letting non-dependent loads go ahead of writes
• what if the load does depend on the write?
• Data dependence speculation
• Value prediction
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Memory Hierarchy
• How many ports on the caches? Main memory?
• Instructions and data?
• How can we fetch so many things in parallel?
• Non-blocking loads, prefetching
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Registers
• How many registers do we need?
• How will this work?
• Renaming, etc.
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Exceptions
• How can we maintain precise exceptions?
• Can we isolate the right instruction in one cycle?
• How quickly do we need to react to exceptions?
• Can we leverage mechanisms from speculation?
• How does this compare with branch speculation?
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